High-Spectral-Efficiency 600-Gbps/Carrier Transmission Using PDM-256QAM Format

High-capacity carrier signals with high-order modulation are being considered for data center interconnection (DCI) applications as they can reduce the cost per bit by decreasing the number of devices and increasing fiber capacity. However, signal quality degradation due to device imperfections affecting the frequency response has limited the capacity per carrier of PDM-256QAM signals to under 400 Gbps. Here, it is possible to use a calibration method for dealing with device imperfections wherein a fixed equalizer separately calibrates the frequency response of the transmitter and receiver. Our previous study showed that PDM-256QAM signals at up to 48 GBaud could be transmitted over 100 km, while it used an experimental setup with total 3-dB analog bandwidth of 13 and 20 GHz for transmitter and receiver, respectively. A micro-integrable tunable laser assembly with a typical linewidth of 20 kHz, maximum under 100 kHz. Actual DCI applications, however, are expected to use dense wavelength division multiplexing (WDM), so in this study, we examined transmission of 10-WDM 48-GBaud PDM-256QAM signals in a 50-GHz grid and evaluated the WDM penalties. The resulting spectral efficiency of 600-Gbps/carrier WDM transmissions was as high as 12.02 bit/s/Hz.